Metallic foams and porous metal structures are valuable for their unique characteristics such as high specific strength, energy absorption at constant crushing load, efficient heat transfer and acoustic properties, all of which can be tailored by controlling the porosity. Many techniques for generating metal foams exist, but the vast majority of metal foam production is through liquid state processes such as the melt processing of aluminum by gas injection or decomposition of a dispersed foaming agent.
Aluminum has dominated the metal foam industry due to its low melting temperature and relative stability in air. However, reactive metals and those with higher melting temperatures require special processing, usually through solid state techniques. In addition, solid state foaming of metals by gas entrapment typically uses a two-step process: (i) entrapment of gas within interparticle voids during powder consolidation; and (ii) heating to expand the entrapped gas within the interparticle voids such that the internal pressure exceeds the yield strength and enables plasticity or creep to increase porosity. As such, the current limitation of solid-state expansion via gas entrapment is controlled by voids formed between solid particles during consolidation, i.e. initial gas pressure and annealing temperatures determine the resulting porosity.
In contrast, if the expanding gas is not limited to gas trapped between particles, but includes gas located within particles, solid state foaming could assume a character more akin to expandable polymers which foam from the constituent pellets. Therefore, an improved solid-state metal foaming process would be desirable.